1. Field of the Invention
The invention relates to a noise removal circuit, and more specifically to a circuit for removing noise from a signal generated by an oscillator.
2. Description of the Related Art
A crystal oscillator generates a signal for generating a clock. The Signal is input to a clock generator generating a clock signal necessary to drive a microprocessor. However, the oscillating signal generated by a crystal oscillator may include noise components due to printed circuit board noise, electromagnetic interference (EMI) and a switching noise of other devices. FIG. 1 is a schematic diagram of a crystal oscillator 110 comprising a resistor Rf, capacitors C1 and C2, inverters INV1 and INV2 and a vibrator 111 including a quartz 112, and quartz input plates 113. The capacitors C1 and C2 are coupled between both terminals of vibrator 111 and ground. A supply voltage VDD is applied to both the inverters INV1 and INV2. The inverter INV1 is coupled to the vibrator 111 and resistor Rf respectively in parallel and generates an oscillating signal xtalmo. The inverter INV2 functions as a buffer and an amplifier to amplify the oscillating signal xtalmo received from the inverter INV1 and convert it to a square wave signal accordingly, generating an output clock signal X1. FIG. 2 shows a waveform of the oscillating signal xtalmo and the output clock signal X1 respectively. As shown, in the initial stage of the oscillation, the oscillating signal xtalmo includes noise components which may thereby affect the output of the inverter INV2, incurring generation of the output clock signal X1 of undesirable frequency. Thus, it is desirable to remove the noise generated in the oscillating initial state, avoiding potential effects on, for example, the microprocessors the output clock signal applied to.